Despite technical advances in some areas, delivery of certain molecules into cells remains a challenge because of factors such as size or charge of the molecule. A plasma or cell membrane is a semi-permeable biological membrane, which acts as a selective barrier, regulating the chemical composition of a cell. Therefore, only certain molecules can translocate across the plasma membrane by passive diffusion into a cell. Small, hydrophobic molecules (such as O2, CO2 and N2) and small, uncharged polar molecules (such as H2O and glycerol) can passively diffuse across a plasma membrane. Larger, uncharged polar molecules (such as amino acids, glucose, and nucleotides) and ions (such as H+, Na+, K+ and Cl−) cannot passively diffuse into a cell.